| Powdery mildew, caused
by the fungus Uncinula necator, can be found in most areas
of the world where grapes are grown. The fungus affects all green
tissues and can be found mostly on the leaves and fruit stems.
Symptoms
Infection on leaves appears conspicuously on the upper surface
as white, powdery patches. Later, the entire leaf surface may be
covered with the fungus. Severe leaf infection can result in cupping
of leaves. Diseased leaves will scorch or turn brown and fall. Affected
berries have a dull, darkened appearance and are usually covered
with the light powdery growth of the fungus. In some varieties new
fruit infection will cease as the sugar content rises and the fruit
matures. When grape cluster stems become covered with the fungus,
the stems shrivel and considerable losses from shelling of the fruit
may occur. Black specks of the overwintering spore-forming bodies
can be seen in the older affected areas. Cluster infections around
bloom may lead to poor fruit set, while slightly later infection
can cause berry splitting. Fruit infection may also reduce wine
quality on varieties intended for that use.
Disease Cycle
The powdery mildew fungus overwinters inside dormant buds and on
the bark of the vine as tiny, black, fruiting bodies (cleistothecia).
Spores (ascospores) contained in the cleistothecia are released
during rains from bud break until shortly after bloom. They are
wind-dispersed to emerging leaves and clusters, and can infect wet
or dry tissue at temperatures of 50 degrees F or higher. Infection
usually starts soon after the blossom period and will continue on
the foliage through the growing season. The fungus develops in dry
conditions when the relative humidity is high. Mildew colonies produce
masses of white, powdery secondary spores (conidia). Conidia are
wind-dispersed throughout the vineyard and do not require rain for
release or infection. New colonies that result from these secondary
infections produce additional conidia, which can continue to spread
the disease. This repeating cycle of infection, spore production,
spore dispersal, and reinfection can continue to occur and reoccur
in as little as 5 to 7 days. Thus, powdery mildew epidemics can
suddenly explode when temperatures are favorable, unless they are
managed efficiently.
Recent studies have demonstrated that berries are highly susceptible
to infection from the immediate prebloom stage until about 2 weeks
after fruit set. Berries of Concord become almost completely resistant
to infection after this time. Concord rachises remain susceptible
until harvest, but the economic importance of mid- or late-summer
rachis infections on processing fruit is questionable. Extensive
splitting of berries and severe fruit damage are almost always the
result of infections that occurred during the immediate prebloom
through fruit set period when berries are most susceptible.
Disease Management
Research has shown that in years when multiple rain events (four
or more) have occurred during the period of maximum fruit susceptibility,
powdery mildew has been a particularly serious problem regionwide.
Thus, for best results, management programs should be at their peak
from prebloom through fruit set, especially if weather is wet. Leaf
infections that occur beyond the fruit set period are much less
serious on Concord and similar cultivars than on V. vinifera
and susceptible hybrids. Such infections do not appear to affect
Concord crop yield or quality. On V. vinifera and highly
susceptible hybrid cultivars, continued suppression of foliar mildew
generally is required at least until veraison to avoid early defoliation.
Good management of late-season leaf infection also reduces disease
pressure the following year by limiting the number of fungal fruiting
structures that overwinter and initiate infection in the spring.
For effective management of powdery mildew, sprays may be required
as early as 1 to 2 inches of shoot growth on V. vinifera
cultivars (depending on rain and temperature) and should not be
delayed beyond the immediate prebloom stage on any cultivar. High
disease levels the previous year increase early season disease pressure
and consequently the importance of early season sprays. Prebloom
through fruit set is the most critical period for managing fruit
infections on all cultivars, and programs should be especially strenuous
if weather is wet during this time. Continued protection through
veraison may be beneficial for attaining maximum fruit quality on
V. vinifera and susceptible hybrid cultivars and is generally
required to prevent premature defoliation.
Cultural practices can reduce disease severity. Plant in sites
with good air circulation and sun exposure. Use a training system
that allows good air movement through the canopy and prevents excess
shading.
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